The major goal of our research program are to construct a model of the genetic control of meiotic recombination, the manner in which meiocytes develop the capacity for enhanced exchange, and the relation of genetic recombination to the control of chromosome segregation and the formation of euploid gametes. To achieve these goals a detailed analysis of the rules of exchange in wild type population of mitotic and meiotic cells of the simple eucaryote, S. cerevisiae, are being pursued, in conjunction with a program to isolate and characterize mutants defective in exchange and segregation. The mutants are detected either by meiotic defects leading to a reduction in the number of gametes formed, the production of inviable gametes, or by specific selection for enhanced or depressed intra- and/or intergenic exchange.